A conical scan (conscan) receiver in its simplest form is a mechanical apparatus that nutates the main lobe of a radar beam about an axis normal to the plane of the antenna. The beam is usually tilted off the axis approximately one-half the 3 db beamwidth. Information indicating the direction of a signal with respect to the axis of the antenna is contained in the amplitude modulated portion of received signals and in the phase relation between the received signal modulation and a reference generated by a device attached to the nutator. This simple form of conscan receiver has been found useful in both missile and radar systems since only a single channel is required to process the received signals.
In recent years, the mechanical nutator and parabolic dish have been replaced by an electronic scanner, wherein the outputs from each element of the array are delayed sinusoidally in ferrite phase shifters. A continuously scanning beam like that of the mechanical conscan receiver, except without the inertia losses inherent in the mechanical system, was thereby obtainable.
With advances in electronic countermeasure technology, the generating of power modulation signals by external sources which could interfere with the received signals came into use. Interference having the same or nearly the same regularity or frequency as the signal modulation frequency could be generated to cause confusion and unreliability in received information.
In an attempt to overcome this regularity problem, a conscan system which changes the rate of beam nutation, or scan, pseudorandomly was developed as disclosed in U.S. Pat. No. 3,859,658. False antenna pointing errors caused by incidental or deliberate amplitude modulation interference at the constant scan frequency of the energy received from the source being tracked were diminished. However, the technique taught by the reference was still susceptible to undesirable interference. This susceptibility can be understood by considering the frequency spectrum over which information is being received. In the use of a radar at a single scan speed the information is carried on one frequency which is easily monitored and interfered with. The pseudorandom varying of scan rate spreads the information over a set bandwidth which, though more difficult, can be subjected to interference. Thus, although an improvement over the prior art, the pseudorandom scan rate was also found to be deficient.
Further, the requirement of fast AGC in prior art receivers has often caused a phase shift in the modulation envelopes of received signals, resulting in cross-coupling between azimuth and elevation error signals. Such crosscoupling has led to erroneous angle information which has seriously degraded the performance of such systems.